Converters for converting a DC voltage in a DC voltage, typically measure the inductor current and the output/capacitor voltage and use these measurements as input for the controller for the DC-DC conversion. As a response to the occurrence of errors in the state is desired, the errors in the state need to be known so besides the measured current state, also the required state should be known, which may consist of the required inductor current and the required capacitor/output voltage. Typically the output voltage corresponds to the state variable which will be controlled, and the value thereof will typically be known. However, the also required inductor current is more difficult to obtain. The relation between the inductor current and other system parameters may differ depending on the particular type of DC-DC converter. However, the inductor current will always depend on the load current. If this load current is not known, it may be considered as a disturbance from the outside of the system.
Instead of determining the particular load current, it may also be possible to determine further currents or voltages within the circuit to determine the required inductor current indirectly or the error in the inductor current directly. For example, in a so-called buck converter, the error in the inductor current will correspond to the capacitor current and can be determined directly.
The load current may be determined by measuring the load current. However, in order to measure the current, a resistor typically is placed in series with the load. If a relatively large resistance is used, accurate measurements will be enabled, but this will on the other hand lead to a greater power dissipation and a greater voltage drop during transients. On the other side, a smaller resistor will make it more difficult to measure the current accurately and a very sensitive and accurate operation amplifier will be required. Furthermore, if the load current is to be determined by measurement, these current measurements of the inductor current and the load current need to be matched as the error in the inductor current is a function of the measured actual inductor current and the load current.
Alternatively, the load current is not measured in several control systems but it can be compensated via a control loop. For the case that the load current is present in the control loop, such a situation is referred to as estimation of the load current. In other words, such an estimator for a disturbance from the outside may be referred to as a disturbance estimator.
FIG. 1 shows a block diagram of a complete converter system according to the prior art. The converter comprises a switching converter unit SCU, an estimator unit EU with a required estimator unit REU and a controller CU. The switching converter SCU comprises three inputs, namely the input voltage Vin, the load current Iload and the duty-cycle d. It should be noted that merely the duty-cycle d of these three inputs can be controlled by the controller CU. The other two inputs are determined by further external elements. The outputs of the switching converter SCU are the inductor current iL and the capacitor voltage vC. These two outputs can be measured and constitute the output signals. The inductor current iL and the capacitor voltage vc are input to the estimator unit EU. Furthermore, the capacitor voltage vc is forwarded to a summation unit SUM where a reference voltage Vref is subtracted from the capacitor voltage VC in order to determine the error evc in the capacitor voltage. In the estimator unit EU the error EIE in the inductor current is determined. Apart from the error evc in the capacitor voltage and the error in the inductor current EIE, also the required capacitor voltage Vcref, the required inductor current iLREF, the input voltage vin and possibly some other parameters may constitute inputs for the controller CU. The controller CU outputs the duty-cycle d as output signal. The estimator EU may receive the inductor current iL, the capacitor voltage vc and the duty-cycle as input signals. The outputs of the estimator unit EU constitute the required inductor current RIE and the error in the inductor current EIE. Accordingly, the controller CU does not need to know the load current as the relation of the required inductor current on the load current is dealt with in the estimator.
In the article “Discrete sliding mode control for DC/DC converters”, Power Electronics Congress, 2000 CIEP 2000, VII IEEE International, 15-19 Oct. 2000, Pages 231-236 by Orosco et al., a DC-DC converter is shown, wherein an estimation of the error in the inductor current is obtained by high pass filtering of the measured inductor current.